Methods of treating of the surface of metal components have been known for a very long time. The aim of such treatments is usually to improve one or more physical properties of the surface of such components e.g. improving hardness, improving corrosion resistance, reducing wear rate, etc.
One way of treating such metal components is to bombard the surface with non-metal ions which alter the surface composition, usually resulting in improved physical properties.
In particular, treating non-ferrous metals e.g. aluminium, titanium, magnesium, zirconium and their alloys with oxygen can give significant improvements in hardness, wear resistance and corrosion resistance due to thickening and strengthening of the naturally occurring surface oxide layer. Numerous methods of producing such regions of oxide have been proposed.
U.S. Pat. No. 6,210,807 discloses a gaseous oxidation method to improve the tribological properties of pure grades of titanium and titanium alloys. Gaseous oxidation is performed at a temperature range of 500 to 725° C. for 0.5 to 100 hours, producing an adherent surface compound layer of 0.2-2 μm on a solid solution-strengthened diffusion zone having a depth of 5 to 50 μm.
Another gaseous oxidation process is described in U.S. Pat. No. 6,833,197, a process referred to as ‘oxygen boost diffusion’, to harden titanium, zirconium and their alloys to depths of 200 to 500 μm. Components are first heat-treated in an oxidizing atmosphere at a temperature range of 700 to 1000° C. and subsequently heat-treated in a vacuum or in a neutral or inert atmosphere at a temperature range of 700 to 1000° C. to promote oxygen diffusion from the oxide layer into the component to produce a sigmoid-shaped hardness profile.
U.S. Pat. No. 5,037,438 discloses orthopedic implants of zirconium or zirconium-based alloy coated with thin blue-black or black zirconium oxide. Zirconium oxide is formed by air oxidation in a furnace or by salt bath oxidation. U.S. Pat. No. 5,370,694 is also directed to the formation of blue-black zirconium oxide, black zirconium oxide and zirconium nitride coatings on zirconium or zirconium alloy prostheses. Zirconium oxide coatings are produced by either air or salt bath oxidation at 550-800° C.
U.S. Pat. No. 5,372,660 discloses a method of surface hardening of titanium alloys with an amount of zirconium sufficient to allow the formation of zirconium oxide at the implant surface. The oxidation process is carried out in an oxygen atmosphere at 200-1200° C.
U.S. Pat. No. 5,415,704 and U.S. Pat. No. 5,498,302 describe methods of hardening implants by diffusing oxygen, nitrogen or carbon into titanium, zirconium and cobalt-based alloys with small concentrations of a metal solute via internal oxidation or nitridation.
Other methods of oxidation currently applied to aluminium, titanium, magnesium and their alloys involve aqueous environments, such as plasma electrolytic oxidation and anodizing.
D.C. diode glow discharges have been used to harden pure titanium and Ti-6Al-4V with oxygen, using air as the treatment atmosphere. Ti-6Al-4V has also been treated using a mixture of argon and oxygen under a D.C. diode glow discharge.